Climate change refers to fluctuations in the Earth’s climate over a long period of time. Defined as the average of global or of a locale’s weather patterns over an extended period of time, climate is different from normal variations in weather, which can change on a regional scale, hour to hour, day to day, season to season.

Current estimates geologically date the Earth to about 4.5 billion years, and it is important to note that the Earth’s climate has changed over time. In more recent history, the northern hemisphere experienced above average temperatures from the eleventh century through the fifteenth century, while the seventeenth through mid-nineteenth century experienced temperatures that were colder than normal. Climate can also vary on a short term basis due to volcanic eruptions or certain shifts in the Earth’s system, for example, El Niño, La Niña, or North Atlantic oscillation patterns.

It is common knowledge that greenhouse gases, such as carbon dioxide, methane, ozone, and water vapor, absorb radiation in the atmosphere, naturally heating the Earth's surface. Without this greenhouse effect, the Earth would be inhabitable for most forms of life. However, scientists are attributing the record rate of warming of the twentieth century and present day to a human activity-enhanced greenhouse effect. As humans create and release more man-made greenhouse gases into the atmosphere, the natural greenhouse effect is magnified, trapping more heat than is released into space and causing more warming of the Earth’s surface.

This amplified level of warming is a concern. Scientists all over the world are studying the occurrence of climate change over the past century and the impacts it will have on the Earth in the future. Calls for adaptation at all levels - internationally, nationally, and locally - are being sounded. The discussion of climate change has been brought out of the laboratory and into the forefront.

Local climate change impacts for the Delaware River Basin include increased temperature, changes in precipitation patterns, and sea level rise. DRBC's State of the Basin Report (2008) included a feature on climate change (in the hydrology section), which highlighted the need for more localized studies, mapping, monitoring, and modeling, as well as for planning initiatives that integrate the reality of a changing climate. Basin water resource managers must seriously look at how climate change will affect the watershed and how to best adapt.